Method and apparatus for gas-lift operations in oil wells

ABSTRACT

A conventional gas-lift system is installed in an oil well at the initial completion and fitting-out of the well. Each gas-lift valve is provided with a blocking device which closes in response to hydrostatic pressure in the casing annulus greater than the dome pressure in the gas-lift valve assembly. Liquids in the casing annulus may be displaced through a sleeve valve in the tubing by gas injection pressure greater than the hydrostatic pressure, after which the gas injection pressure may be reduced to open the blocking devices for gas flow into the tubing string.

United States Patent Inventors Everett D. McMurry;

Bolling A. Abercrombie, both of Houston,

Tex.

Sept. 4, 1970 Dec. 28, 1971 McMurry Oil Tools, Inc. Houston, Tex.

Appl. No. Filed Patented Assignee Continuation of application Ser. No. 820,454, Apr. 30, 1969, now abandoned. This application Sept. 4, 1970, Ser. No.

METHOD AND APPARATUS FOR GAS-LIFT OPERATIONS IN OIL WELLS 28 Claims, 6 Drawing Figs.

U.S. Cl

Int. Cl

Field of Search 417/54, 137/155 F041l/l8, F04f l/08 417/111,

[56] References Cited UNITED STATES PATENTS 2,312,315 3/1943 Boynton 137/155 2,342,301 2/1944 Peters 137/155 3,010,406 11/1961 Vincent 417/113 Primary Examiner-Robert M. Walker Attorneys-Arnold, White & Durkee, Tom Arnold, Bill Durkee, Frank S. Vaden Ill and Stanley A. Becker ABSTRACT: A conventional gas-lift system is installed in an oil well at the initial completion and fitting-out of the well. Each gas-lift valve is provided with a blocking device which closes in response to hydrostatic pressure in the casing annulus greater than the dome pressure in the gas-lift valve assembly. Liquids in the casing annulus may be displaced through a sleeve valve in the tubing by gas injection pressure greater than the hydrostatic pressure, after which the gas injection pressure may be reduced to open the blocking devices for gas flow into the tubing string.

SHEET 1 0F 2 INVENTORS I lmgu Z Uwlbee ATTORNEYS Everett DfMcMurry 8 Bolling A.Abercrombie BY AWM,

METIIDD AND APPARATUS FOR GAS-LEM OPERATIONS IN OIL WELLS This application is a continuation of application Ser. No. 820,454, filed Apr. 30, 1969, now abandoned.

BACKGROUND OF INVENTION This invention relates to methods and apparatus for producing oil from subsurface earth formations, and more particularly relates to improved gas-lift methods and apparatus.

It is well known that the formation pressure is often initially great enough to overcome the hydrostatic pressure of the oil in the well bore, and that the oil in the formation will therefore flow to the surface without the assistance of artificial lifting methods and apparatus. However, it is also well known that the formation pressure often declines as oil is recovered, until the formation pressure is insufficient to overcome the hydrostatic pressure in the well bore, whereupon pumping or gas-lift techniques must then be employed.

A great deal of time and expense could be saved if it was possible to install the gas-lift valve assemblies in the tubing string at the time the tubing string is initially run into the borehole and perforated. However, it is common practice to load the casing annulus with drilling mud and other high density liquids commonly known as kill fluid" during the period when the formation pressure is used to lift the oil to the surface. The kill fluid must normally be ejected or displaced from the casing annulus before gas-lift operations may be commenced. This is-usually accomplished by circulating a clean fluid such as oil or salt water down into the casing annulus from the surface, to drive the kill fluid down the casing annulus and up through the tubing to the surface. It will be apparent that the injection pressure required to displace the kill fluid will necessarily be greater than the hydrostatic pressure of the liquid in the casing annulus, and thus will greatly exceed the dome pressure in the gas-lift valves. Accordingly, if the gas-lift valves have been preinstalled, these valves will open under the pressure applied to displace the kill fluid, and this will cause the liquids to flow through the gas-lift valves into the tubing. This, in turn, tends to abrade the shutofi surfaces of the gas-lift valves.

Because of this disadvantage, it is common practice to preinstall sidepocket" mandrels in the tubing string, such as depicted in U.S. Pat. No. 2,664,162, and to plug each mandrel with a dummy valve or other removable plugging device. Thus, when it becomes necessary to shift to gas-lift at a later time, the liquid is first ejected from the casing annulus as hereinbefore described, and the dummy valves are thereafter removed and replaced with actual gas-lift valve assemblies.

The advantage of the foregoing technique is that preinstalling the mandrels saves the time and expense of subsequently removing the tubing string from a producing well for the purpose of installing the mandrels. However, once the dummy valves are installed in the well bore, they must be removed and replaced with operable gas-lift valves by means of wire-line techniques which are also time consuming and expensive, especially in submerged offshore wells.

SUMMARY OF THE INVENTION These disadvantages of the prior art are overcome with the present invention, and methods and apparatus are provided herewith for selectively blocking fluid flow through preinstalled gas lift valves while displacing the kill fluid from the casing annulus and for thereafter selectively opening fluid flow between the annulus and the tubing string and placing the well into production by conventional gas-lift methods and apparatus.

In a preferred embodiment of the present invention, the tubing string is preequipped with conventional gas-lift valves each having a normally open blocking device which closes when the casing pressure exceeds a certain level greater than the actuating or dome pressure of the gas-lift valve. Thus, if the hydrostatic pressure of the kill fluid in the annulus is great enough to open the gas-lift valve, the blocking device will close to prevent kill fluid from flowing through the gas-lift valve and thereby ruin its shutoff surfaces.

The tubing string is also preferably preequipped with an openable section such as a sleeve valve at'a depth below the lowest gas-lift valve assembly in the well bore, and in some in-- stances, may be provided with an upwardly openable check valve or plug of suitable design located at a depth below the sleeve valve. When it is desired to shift to gas-lift operations in a well which has been preequipped in this manner, the sleeve valve is opened and clean liquid may then be injected into the top of the annulus to drive the kill fluid down through the easing annulus and into the tubing string by way of the open sleeve valve. The check valve or plug blocks downward fluid flow in the tubing string, and thus all fluid entering through the sleeve valve will flow to the surface. The. casingannulus may now be filled with a suitable flushing fluid to clean the annulus of debris, and the flushing fluid is then also driven out of the. annulus, and to the surface, by means of injection gas.

The amount of pressure required to displace the kill fluid and flushing fluid will depend on the height of the fluid inthe well bore. Once the level of the fluid is driven below the gaslift valves, however, the pressure of the injection gas need only be high enough to open the gas-lift valves since the displaced fluid will not flow back into the annulus.

The closing pressure established for each blocking device is greater than the dome pressure of the gas-lift valve associated with the blocking device. Thus, gas will initially be injected at a pressure greater than the peak hydrostatic pressure, to drive the liquid level in the casing annulus down below the top gas lift valve assembly. When the liquid level drops below the uppermost valve assembly, the injection pressure may then be reduced until it is lower than the pressure which will close the blocking device, but still higher than the dome pressure of the gas-lift valve. Accordingly, gas-will now flow through both the gas-lift valve and the blocking device, and into the: tubing to help carry the liquids therein to the surface. Thereafter, the injection pressure may be raised sufficiently, or maintained, to continue depressing the level of the liquid in the casing-annulus, but not great enough to close the uppermost blocking device. Each blocking device will thereafter open as the level of the annulus liquid drops past it (since the actuating pressureof each blocking device is preselected to correlate with the dome pressure of the uppermost blocking device or its own associated gas lift valve), until all of the gas lift valves required for performance can conduct gas into the tubing from the easing. At this stage, the well has been shifted over to gas-lift operation.

Although the foregoing description has been directed to a well wherein the casing annulus is used as the injection conduit in the well bore, wells may be equipped in a manner such that the casing annulus provides the production conduit and the tubing string functions as the injection conduit. This difference is immaterial to the basic concepts of the present invention, however.

These and other features of the present invention will become apparent from the following detailed description, wherein reference is made to the figures of the accompanying drawings.

IN THE DRAWINGS FIG. 1 is a simplified pictorial representation of a typical oil and gas well having a plurality of conventional gas-lift valve assemblies arranged along the length of the tubing string installed in the well, and including provision for opening such valves by injecting gas pressure into the annulus between the tubing string and the casing.

FIG. 2 is a simplified functional representation of one embodiment of the present invention, including the fundamental parts of a conventional gas-lift valve and. a blocking valve associated therewith.

FIG. 3 is a simplified functional representation of a similar but different embodiment of the invention as depicted in FIG. 2.

FIG. 4 is a detailed pictorial representation, partly in cross section, of a portion of the apparatus depicted in FIG. 2.

FIG. 5 is a detailed pictorial representation, partly in cross section, of another portion of the embodiment depicted in FIG. 2. In the posture shown, the lower end of the FIG. 4 portion joins the top of the FIG. 5 portion.

FIG. 6 is a pictorial representation of an alternative form of a portion of the apparatus depicted in FIG. 1.

DETAILED DESCRIPTION Referring now to FIG. 1, there may be seen a simplified pictorial representation of an oil well equipped according to the principles of the present invention as hereinbefore described. In particular, the borehole 50 contains a casing 51 and a tubing string 52 interconnected at the surface by a conventional wellhead assembly 60. An injection line 59 is connected to the wellhead assembly 60 to provide injection gas to the annulus 55, and a production line 58 may be connected in a conventional manner to receive fluid from the tubing string 52.

As also hereinbefore explained, the tubing string 52 may be preequipped with a plurality of conventional mandrels 53 at suitable preselected locations along its length. A conventional sleeve valve 54 may be located below the lowest mandrel 53, and above a packer 57 or other suitable means for sealing the annulus 55, and a plug or upwardly opening check valve 56 of any suitable design is preferably mounted in the tubing string 52 below the sleeve valve 54 and packer 57.

In alternative systems, commonly used but not illustrated, a conventional check valve may be used in conjunction with the sleeve valve 54, so as to prevent backflow from the tubing 52 into the annulus 55. In still another common alternative system that is not illustrated, there may be no packer at all, with the result that circulation around the lower end of the tubing 52, is free in both directions between the annulus 55 and the tubing 52 subject to the forces of inertia of the liquid and pressure in the injection conduit, here being the annulus 55, and the production conduit, here being the tubing 52. While the invention of this patent may be used with these and other alternative systems, the invention is herein described in conjunction with the system illustrated in FIG. 1.

Referring now to FIG. 2, there may be seen a simplified functional representation of one form of apparatus embodying the concept of the present invention and suitable for installation and use on any one of the mandrels 53 illustrated in FIG. 1. In particular, the apparatus may comprise a tubular housing 61 for providing a fluid flow path, and including an intake port 64 located to receive pressure from either the annulus 55 or the tubing string 52 in FIG. 1, or from any other injection conduit which may be provided in a well bore. As further illustrated in FIG. 2, fluid flow entering the housing 61 from the intake port 64 is directed through the housing 61 to either the tubing string 52 or the casing annulus S5, or to any other production conduit which may be provided in a well bore as hereinbefore explained.

As depicted in FIG. 2, the housing 61 may include a conventional gas-lift valve 62 arranged downstream of the intake port 64, and a blocking assembly 63 arranged in the housing 61 downstream of the gas-lift valve 62. The gas-lift valve 62 is preferably responsive to pressure in the injection conduit, and thus it may include a conventional bellows 67 arranged upstream of an annular seat member 65 to retract or extend a suitable shutofl' member 66 to and from engagement with the seat member 65 depending on whether the pressure in the injection conduit is above or below the dome pressure in the bellows 67.

The blocking assembly 63 may be seen to include an annular seat member 68 located downstream of the gas-lift valve 62, and a shutoff member 69 extending through the seat member 68 and fixedly attached to a bellows 70 located downstream of the seat member 68. The dome pressure in the bellows 70 is preferably greater than the dome pressure in the other bellows 67, but less than a predetermined pressure corresponding to the minimum hydrostatic pressure in the injection conduit. Accordingly, the dome pressure in the bellows is sufficient to keep the bellows 70 distended longitudinally in the housing 61 at all times except when the hydrostatic pressure reaching the bellows 70, as for example through the seat members 65 and 68, exceeds the dome pressure, whereupon the bellows 70 will contract and pull the shutoff member 69 into closing engagement with the seat member 68.

Referring now to FIG. 3, there may be seen a similar functional representation of an alternative embodiment of the apparatus illustrated in general terms in FIG. 2, wherein the relative locations of the gas-lift valve 62 and blocking device 63 have been reversed in the housing 61. In particular, the seat member 68 is now located upstream of the seat member 65, and the bellows 67 of the gas-lift valve 62 is now subject to pressure downstream of the blocking device 63. In this arrangement, the normally open blocking device 63 will admit fluid pressures to the bellows 67 in excess of the dome pressure therein, and thus the gas-lift valve 62 will open if the pressure in the injection conduit is greater than the dome pressure in its bellows 67 but less than the dome pressure in the bellows 70 of the blocking device 63. However, it should be noted that, in the apparatus depicted in FIG. 3, the gas-lift valve 62 will tend to close if the pressure in the injection conduit rises above the dome pressure in the bellows 70 and closes the blocking device 63. This will occur if the pressure in the production conduit is less than the dome pressure in the bellows 67, since the pressure trapped downstream of the seat member 68 will decline after the blocking device 63 closes until it equals or substantially equals the dome pressure in the bellows 67.

Referring now to. FIGS. 4 and 5, there may be seen a detailed pictorial representation, partly in cross section, of the apparatus depicted in FIG. 2. In particular, the gas-lift valve assembly 62 includes a housing 2 having an intake port 3, and further including a valve stem 42 having a head 43 adapted for sealing engagement with an annular valve seat member 4. The stem 42 is mounted at one end of a conventional bellows 44 located upstream of the seat member 4 and having a dome pressure sufficient to urge the head 43 in sealing engagement with the seat member 4 except when subjected to ambient pressures sufficient to contract the bellows 44 and to draw the stem 42 and head 43 out of shutoff engagement with the seat member 4. I s

As may further be seen, the blocking assembly 63 may include a lower housing section 8 threadedly engaged at one end with the upper end of a seat receiver 7 which, in turn, may be threadedly engaged at its lower end with the upper end of the housing 2. The gas-lift valve seat member 65 depicted in FIG. 2 may be seen in FIG. 5 to include an annular seat member 4 which is slidably disposed in the lower end of a seat receiver 7, and which may be held in place therein by any suitable means such as a snap ring 5. A conventional O-ring gasket 6 may be included therebetween to provide a gastight seal between the outer circumference of the seat member 4 and the inside surface of the seat receiver 7. Similarly, an O-ring gasket 9 may be included to provide a gastight seal between the outer circumference of the seat receiver 7 and the inside surface of the lower housing section 8.

A lower check valve seat member 10 is preferably slidably disposed in the upper end of the seat receiver 7, and may be held in place therein by any suitable means such as a snap ring 12. An O-ring gasket 11 or other suitable sealing means may be included as illustrated to provide a gastight seal between the lower check seat member 10 and the seat receiver 7.

A lower check stem body 14, having a generally annular or hollow cylindrical configuration, and also having a rounded shutoff head portion 13, with one or more passageways 15 as illustrated, may be disposed within the lower end of the lower housing section 8 for shutoff engagement with the upper end of the lower check seat member 10. A suitable check spring 16 may be disposed under compression between the upper surface of the lower check valve stem member 14 and the upper annular internal shoulder portion of the lower housing section 8, to keep the lower check valve normally closed against backflow.

The upper portion of the lower housing section 5 may be adapted for threaded engagement with the lower end of the upper housing section 17, and may also be adapted to support an annular blocking seat member 19 threadedly disposes therein. A suitable O-ring gasket or other sealing means may be interconnected between the lower housing section 5 and the upper housing section 117.

Gas flow through the blocking seat member 19 may be controlled by a blocking stem 20 located below the seat member 19 and having a shank portion extending upwardly through the seat member 119, The threaded upper end of the stem 20 may be threadedly connected to a stem holder 2 which is disposed within a blocking seat member 119, and which may have a bypass port 22 as illustrated. As further illustrated, the blocking seat member 19 may have one or two bypass ports 211 and 23 for receiving gas from its lower annular end, and for conducting such gas or gas pressure upwardly about its outside circumference and into the annular space adjacent the inside surface of the upper housing section 17.

The stem holder 24 may be threadedly engaged with the lower end of a tubular bellows guide 26, and a conventional bellows 25 may be disposed concentrically about the bellows guide 26 for gastight engagement with the stem holder 24 at one end, and with its upper end in gastight engagement with the lower end of a suitable tubular bellows retainer 45. The lower end of the bellows retainer 45 may be threadedly engaged with the upper end of the blocking seat member 119, and may have its upper end stoppered by a suitable pressure fitting 28 threadedly engaged therewith and adapted for injection of a suitable dome pressure into the bellows 25. A fail-safe spring 27 may be disposed about the upper end of the bellows guide 26, and compressed between the upper shoulder portion of the bellows guide 26, and the lower internal annular shoulder portion of the bellows retainer 415. The purpose of the fail-safe spring 27 is to draw the blocking stem 20 into shutoff engagement with the seat member 19, if the bellows 25 is ruptured and if the dome pressure therein escapes. A similar spring may be advantageously installed under compression about the stem 42 of the gas-lift valve assembly 62 for the same purpose.

As may further be seen in FIG. 4, the upper end of the upper housing section 117 may be threadedly engaged with the lower end of a suitable seat receiver 36 having its upper end threadedly interconnected with a conventional adapter 46. 0- ring gaskets 33 and 47 may be included therewith to provide for a gastight fitting therebetween, and an upper check seat member 51 may be slidably disposed in the upper end of the seat receiver 30 and secured thereto by any suitable means such as a snapring 3d. An O-ring gasket 32 may provide a gastight seal between the seat retainer 30 and the upper check seat member 3i.

An annular upper check stem body 35, having a suitable head 36 and ports 37, may be positioned adjacent a suitable upper check spring 38 for sealing engagement with the upper check seat member 31. An outlet port 59 may be provided in the upper end of the adapter l6 for communication with pressure in the tubing or other production conduit.

Referring now to FIG. 6, there may be seen an alternative embodiment of the portion of the apparatus depicted in FIG.

interconnected with a manually (wire line or hydraulically), actuated bypass valve 1154 to control fluid flow from the annulus 155 of the casing 151 into the tubing string 1152. Thus, the bypass valve 154 (which may be a wire-line operated valve such as depicted in FIG. ll) may be opened or closed without admitting fluid flow into the tubing 152, provided the pressure in the annulus 155 is less than the dome or actuating pressure of the gas-lift valve 1153.

in this configuration, the manually actuated bypass valve 154 may be normally kept open, and input to the tubing 1152 is controlled entirely by means of the pressure in the annulus 155. Thus, the need for expensive and time-consuming wire- 1, wherein a gas-lift valve 153 of conventional design may be line operations is reduced. Of course, the bypass valve 154' may be omitted, and only the gas-lift valve 153 used. This will eliminate all need for wire-line operations.

it is apparent that in the embodiment as illustrated in FIG. 2 or FIGS. 4-5, for example, there is a first potential flow path between the casing annulus 55 and the inside of the tubing string 52, through at least one assembly including a gas-lift valve 62 and a blocking device 63 in the mandrel 53. A second or parallel potential flow path exists in the sleeve valve 54; When a well is loaded with kill fluid, the kill-fluid hydrostatic head pressure, which may reach the blocking device 63 from either or both tubing and annulus depending upon circumstances, is sufficient to hold the blocking devices 63 in all the mandrel assemblies 53 in closed position. The first flow path is thereby closed.

When such a well is to be placed in gas-lift operation, the sleeve valve 54, which is the second flow path, is opened.

Clean liquid may then be injected through line 59 into the casing annulus 55, forcing the kill fluid down the annulus, through the second flow path of sleeve valve 54 and out the tubing 52. The check valve 56, when it is used, functions to block downward flow in the tubing string. In installations not including such a check valve 56, some flow down into the formation against formation pressure may occur but the function of unloading the injection conduit of kill fluid, is effected nevertheless.

if desired, further flushing fluid may then be injected into the annulus 55 in similar manner to clean out any remaining debris.

The cleaning or flushing fluid is then driven out of the annulus 55 via the second flow path of sleeve valve 54, by gas injected into the annulus 55. This fluid ispreferably driven out at least to a level below the uppermost mandrel 53, such that, in the event of equalization of fluid levels in the annulus 55 and tubing 52, the uppermost mandrel 53 remains above fluid level. Such full equalization is not necessary, but may occur either by exposing the top of tubing 52 to the same pressure as that applied to the annulus 55, or by bleeding the annuluspressure down to the pressure at the top of the tubing 52.

in all events, an adequate amount of fluid is removed from the system to effect a sufficiently reduced load of fluid in the production tubing 52 to permit commencement of gas-lift operations. Now by manipulation of the gas pressures on tubing 52 and annulus 55, gas will flow through both valve openings 65 and 68 putting the well on conventional gaslift. For example, the pressure in the annulus 55 and tubing 52 may be bled down together, to a level whereby the blocking device 63 of at least some uncovered mandrel 53 opens. The well will then further unload and produce in the normal manner for the design of the system, as long as the pressure in the annulus 55 is controlled at less than the preselected pressure that would close the blocking device'63, the gas injection of course occurring at annulus pressures at least as great as the preselected pressure which effects opening of the gas-lift valve 62.

it will be apparent from the foregoing that various modifications and variations may be made in the structures and" procedures described herein without substantial departure from the essential concept of the present invention. Ac-

cordingly, it should be clearly understood that the forms of the invention described herein and depicted in the accompanying drawings, are exemplary only and are not intended as limitations on the scope of the present invention.

What is claimed is:

l. A gas-lift system for use in a well bore containing a production conduit and aninjection conduit, comprising a hollow body member having an intake portion'andan out let portion for providing a flow pathbetween said con duits, a first normally open valve means disposed insaid body member and closeable in responseto pressuresin said im jection conduit greater tliana first-preselected pressure to block said flow path, and

a second normally closed valve means disposed in said body member and openable in response to pressures in said injection conduit greater than a second preselected pressure.

2. A gas-lift system for use in a well bore containing a production conduit and an injection conduit, comprising a hollow body member having an intake portion and an outlet portion for providing a flow path between said'conduits,

a first normally open valve means disposed in said body member and closeable in response to pressures in said injection conduit greater than a first preselected pressure, and

a second normally closed valve means disposed in said body member and openable in response to pressures in said injection conduit greater than a second preselected pres sure, said second preselected pressure being less than said first preselected pressure.

3. The system described in claim 2, wherein said first valve means includes a first seat member disposed in said body member intermediately of said intake and outlet portions thereof,

a first shutofi" member for shutoff engagement with said first seat member to interrupt fluid flow through said body member, and

first driving member for urging said first shutoff member from said first seat member in response to a dome force substantially equal to said first preselected pressure.

4. The system described in claim 3, wherein said first driving member is responsive to pressures in said injection conduit greater than said first preselected pressure to longitudinally contract and draw said first shutoff member into sealing engagement with said first seat member.

5. The system described in claim 4, wherein said first seat member is located intermediately of said first driving member and said intake portion of said body member.

6. The system described in claim 5, wherein said first seat member is an annular body member disposed about said flow path in said hollow body member,

and wherein said first shutoff member is an elongate member extending through said annular body member and having one end fixed to said first driving member and having a shutoff head portion at the other end positioned intermediately of said annular body member and said intake portion of said hollow body member for fluidtight engagement with said annular body member.

7. The system described in claim 6, wherein said first driving member is longitudinally distended by said dome force therein to unseat said head portion of said first shutoff member from said annular body member when said pressures in said injection conduit are less than said first preselected pressure.

8. The system described in claim 7, wherein said first driving member is longitudinally compressed by pressures in said injection conduit greater than said first preselected pressure to draw said head portion of said first shutoff member into fluidtight engagement with said annular body member.

9. The system described in claim 8, wherein said first valve means is located in said hollow body member intermediately of said intake portion thereof and said second valve means.

10. The system described in claim 8, wherein said second valve means is located in said hollow body member intermediately of said intake portion thereof and said first valve means.

11. The system described in claim 10, further including check means for closing said hollow body member in response to pressure in said production conduit greater than pressure in said injection conduit.

12. The check means described in claim 11, comprising first normally closed check valve assembly interconnected in said body member between said first and second seat members and openable in response to pressure in said injection conduit greater than pressure in said production conduit.

13. The check means described in claim 12, further comprising second nonnally closed check valve assembly interconnected in said body member between said outlet portion thereof and said first seat member.

14. The system described in claim 13, wherein said first valve means further comprises spring means for urging said head portion of said first shutoff member into engagement with said annular body member.

15. The system described in claim 8, further comprising a third bypass valve means arranged in said well bore below said hollow body member for intercommunication between said conduits,

plugging means in said injection conduit below said bypass valve means, and

an upwardly openable check valve means disposed in said production conduit below said bypass valve means.

16. The system described in claim 15, wherein said bypass valve means comprises a normally closed control valve openable in response to a preselected third pressure in said injection conduit.

17. The system described in claim 15, wherein said bypass valve means comprises a manually openable valve means.

18. The system described in claim 17, wherein said bypass valve means further includes a normally closed control valve openable in response to a preselected third pressure in said injection conduit.

19. The system described in claim 18, further including another upwardly openable check valve means disposed in said production conduit above said bypass valve means.

20. In a gas-lift valve assembly for use in a well bore containing a production conduit and an injection conduit and including a hollow body member having an intake portion for communication with said injection conduit and an outlet portion for communication with said production conduit,

a first seat member disposed in said body member,

a first shutoff member disposed in said body member, and

a first pressure-responsive member urging said first shutoff member into engagement with said first seat member and responsive to pressures in said injection conduit greater than a first preselected pressure to disengage said first shutoff member from said first seat member,

the improvement in combination therewith comprising blocking means responsive to pressures in said injection conduit greater than a second preselected pressure greater than said first preselected pressure for blocking fluid flow through said hollow body member.

21. The improvement described in claim 20, wherein said blocking means passes fluid flow through said hollow body member when the pressures in said injection conduit are greater than said first preselected pressure but not greater than said second preselected pressure.

22. The blocking device described in claim 21, further comprising a second seat member interconnected with said hollow body member,

a second movable shutoff member for engaging said second seat member, and

a driving means responsive to a dome force substantially equal to said second preselected pressure for urging said second shutoff member from said second seat member.

23. The blocking device described in claim 22, wherein said driving means moves said second shutoff member toward said second seat member in response to pressures in said injection conduit greater than said second preselected pressure.

24. A method of lifting liquids through a well bore containing a production conduit and a liquid-filled injection conduit, comprising establishing a first flow path between said conduits at a first preselected depth in said well bore,

establishing a second flow path between said conduits at asecond depth in said well bore below said first flow path,

opening said second flow path while maintaining said first flow path closed,

injecting gas into said injection conduit at a first pressure sufficient to displace the liquid in said injection conduit downward and through said second flow path into said production conduit,

opening said first flow path after said liquid in said injection conduit has been displaced below said first flow path, and injecting gas through said first flow path into said production conduit.

25. The method described in claim 24, wherein said gas is injected through said first flow path into said production conduit at a second pressure less than said first pressure.

26. The method described in claim 25, wherein said first pressure is greater than the hydrostatic pressure in said liquidfilled injection conduit adjacent said first path, and

wherein said second pressure is less than said first pressure but greater than the pressure in said production conduit. 27. The method described in claim 26, including the step of reducing the pressure of said gas injected into said injection conduit to said second pressure after said liquid in said injection conduit has declined below said first flow path, and

opening said first flow path in response to said reduction of said gas pressure in said injection conduit below said first pressure. 28. The method described in claim 27, including the step of closing said second flow path afier opening said first flow path in said well bore. 

1. A gas-lift system for use in a well bore containing a production conduit and an injection conduit, comprising a hollow body member having an intake portion and an outlet portion for providing a flow path between said conduits, a first normally open valve means disposed in said body member and closeable in response to pressures in said injection conduit greater than a first preselected pressure to block said flow path, and a second normally closed valve means disposed in said body member and openable in response to pressures in said injection conduit greater than a second preselected pressure.
 2. A gas-lift system for use in a well bore containing a production conduit and an injection conduit, comprising a hollow body member having an intake portion and an outlet portion for providing a flow path between said conduits, a first normally open valve means disposed in said body member and closeable in response to pressures in said injection conduit greater than a first preselected pressure, and a second normally closed valve means disposed in said body member and openable in response to pressures in said injection conduit greater than a second preselected pressure, said second preselected pressure being less than said first preselected pressure.
 3. The system described in claim 2, wherein said first valve means includes a first seat member disposed in said body member intermediately of said intake and outlet portions thereof, a first shutoff member for shutoff engagement with said first seat member to interrupt fluid flow through said body member, and first driving member for urging said first shutoff member from said first seat member in response to a dome force substantially equal to said first preselected pressure.
 4. The system described in claim 3, wherein said first driving member is responsive to pressures in said injection conduit greater than said first preselected pressure to longitudinally contract and draw said first shutoff member into sealing engagement with said first seat member.
 5. The system described in claim 4, wherein said first seat member is located intermediately of said first driving member and said intake portion of said body member.
 6. The system described in claim 5, wherein said first seat member is an annular body member disposed about said flow path in said hollow body member, and wherein said first shutoff member is an elongate member extending through said annular body member and having one end fixed to said first driving member and having a shutoff head portion at the other end positioned intermediately of said annular body member and said intake portion of said hollow body member for fluidtight engagement with said annular body member.
 7. The system described in claim 6, wherein said first driving member is longitudinally distended by said dome force therein to unseat said head portion of said first shutoff member from said annular body member when said pressures in said injection conduit are less than said first preselected pressure.
 8. The system described in claim 7, wherein said first driving member is longitudinally compressed by pressures in said injection conduit greater than said first preselected pressure to draw said head portion of said first shutoff member into fluidtight engagement with said annular body member.
 9. The system described in claim 8, wherein said first valve means is located in said hollow body member intermediately of said intake portion thereof and said second valve means.
 10. The system described in claim 8, wherein said second valve means is located in said hollow body member intermediately of said intake portion thereof and said first valve means.
 11. The system described in claim 10, further including check means for closing said hollow body member in response to pressure in said production conduit greater than pressure in said injection conduit.
 12. The check means described in claim 11, comprising first normally closed check valve assembly interconnected in said body member between said first and second seat members and openable in response to pressure in said injection conduit greater than pressure in said production conduit.
 13. The check means described in claim 12, further comprising second normally closed check valve assembly interconnected in said body member between said outlet portion thereof and said first seat member.
 14. The system described in claim 13, wherein said first valve means further comprises spring means for urging said head portion of said first shutoff member into engagement with said annular body member.
 15. The system described in claim 8, further comprising a third bypass valve means arranged in said well bore below said hollow body member for intercommunication between said conduits, plugging means in said injection conduit below said bypass valve means, and an upwardly openable check valve means disposed in said production conduit below said bypass valve means.
 16. The system described in claim 15, wherein said bypass valve means comprises a normally closed control valve openable in response to a preselected third pressure in said injection conduit.
 17. The system described in claim 15, wherein said bypass valve means comprises a manually openable valve means.
 18. The system described in claim 17, wherein said bypass valve means further includes a normally closed control valve openable in response to a preselected third pressure in said injection conduit.
 19. The system described in claim 18, further including another upwardly openable check valve means disposed in said production conduit above said bypass valve means.
 20. In a gas-lift valve assembly for use in a well bore containing a production conduit and an injection conduit and including a hollow body member having an intake portion for communication with said injection conduit and an outlet portion for communication with said production conduit, a first seat member disposed in said body member, a first shutoff member disposed in said body member, and a first pressure-responsive member urging said fiRst shutoff member into engagement with said first seat member and responsive to pressures in said injection conduit greater than a first preselected pressure to disengage said first shutoff member from said first seat member, the improvement in combination therewith comprising blocking means responsive to pressures in said injection conduit greater than a second preselected pressure greater than said first preselected pressure for blocking fluid flow through said hollow body member.
 21. The improvement described in claim 20, wherein said blocking means passes fluid flow through said hollow body member when the pressures in said injection conduit are greater than said first preselected pressure but not greater than said second preselected pressure.
 22. The blocking device described in claim 21, further comprising a second seat member interconnected with said hollow body member, a second movable shutoff member for engaging said second seat member, and a driving means responsive to a dome force substantially equal to said second preselected pressure for urging said second shutoff member from said second seat member.
 23. The blocking device described in claim 22, wherein said driving means moves said second shutoff member toward said second seat member in response to pressures in said injection conduit greater than said second preselected pressure.
 24. A method of lifting liquids through a well bore containing a production conduit and a liquid-filled injection conduit, comprising establishing a first flow path between said conduits at a first preselected depth in said well bore, establishing a second flow path between said conduits at a second depth in said well bore below said first flow path, opening said second flow path while maintaining said first flow path closed, injecting gas into said injection conduit at a first pressure sufficient to displace the liquid in said injection conduit downward and through said second flow path into said production conduit, opening said first flow path after said liquid in said injection conduit has been displaced below said first flow path, and injecting gas through said first flow path into said production conduit.
 25. The method described in claim 24, wherein said gas is injected through said first flow path into said production conduit at a second pressure less than said first pressure.
 26. The method described in claim 25, wherein said first pressure is greater than the hydrostatic pressure in said liquid-filled injection conduit adjacent said first path, and wherein said second pressure is less than said first pressure but greater than the pressure in said production conduit.
 27. The method described in claim 26, including the step of reducing the pressure of said gas injected into said injection conduit to said second pressure after said liquid in said injection conduit has declined below said first flow path, and opening said first flow path in response to said reduction of said gas pressure in said injection conduit below said first pressure.
 28. The method described in claim 27, including the step of closing said second flow path after opening said first flow path in said well bore. 